Directed evolution has been used to generate pseudotyped adeno-associated virus (AAV) capsids with novel tropism, reduced non-specific delivery, and immune evasion (Kwon and Schaffer, 2008; Maheshri et al., 2006). Biopanning of phage display and whole viral libraries in vitro and in vivo with different selective pressures has generated proteins with desired properties such as improved ligand binding and uptake, immune interactions, or enzyme activities (Yuan et al., 2005). Thus, this methodology can be a powerful and more effective alternative to rational mutation for creating new protein variants with improved features. The most recently developed approach to this process employs error-prone PCR and staggered extension to create a library of variants that can be screened against different cell types to select out cell-specific targeted vectors (Zhao et al., 1998). To date, a process has not yet been developed to isolate protein variants with improved intracellular trafficking functions. This invention introduces a new procedure and new protein molecules with improved cell penetration functions for drug delivery resulting from the procedure.